Micro Fungi mostly found on leaves (PDF). Rusts
The Rust Fungi of the British Isles (can be found on this link). Rust Fungi
A quick learning key for the macroscopic identification of Waxcaps (PDF). Waxcaps
Keys for Macrofungi. Fungi

Heathland and grassland lichens: a key to south-eastern English Cladonia (PDF). Cladonia

Various can also be found here, worth checking the site regularly for test keys.